Date:   Thu, 29 Sep 2022 16:56:07 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Pierre Gondois <pierre.gondois@....com>
Cc:     linux-kernel@...r.kernel.org,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Valentin Schneider <vschneid@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Will Deacon <will@...nel.org>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v1 1/1] arm_pmu: acpi: Pre-allocate pmu structures

On Thu, Sep 29, 2022 at 04:08:19PM +0200, Pierre Gondois wrote:
> Hello Mark,
> 
> On 9/28/22 17:47, Mark Rutland wrote:
> > Hi Pierre,
> > 
> > Thanks for this, and sorry for the delayed reply.
> > 
> > On Mon, Sep 12, 2022 at 05:51:04PM +0200, Pierre Gondois wrote:
> > > On an Ampere Altra,
> > > Running a preemp_rt kernel based on v5.19-rc3-rt4 on an Ampere Altra
> > > triggers:
> > > [   12.642389] BUG: sleeping function called from invalid context at kernel/locking/spinlock_rt.c:46
> > > [   12.642402] in_atomic(): 0, irqs_disabled(): 128, non_block: 0, pid: 24, name: cpuhp/0
> > > [   12.642406] preempt_count: 0, expected: 0
> > > [   12.642409] RCU nest depth: 0, expected: 0
> > > [   12.642411] 3 locks held by cpuhp/0/24:
> > > [   12.642414] #0: ffffd8a22c8870d0 (cpu_hotplug_lock){++++}-{0:0}, at: cpuhp_thread_fun (linux/kernel/cpu.c:754)
> > > [   12.642429] #1: ffffd8a22c887120 (cpuhp_state-up){+.+.}-{0:0}, at: cpuhp_thread_fun (linux/kernel/cpu.c:754)
> > > [   12.642436] #2: ffff083e7f0d97b8 ((&c->lock)){+.+.}-{3:3}, at: ___slab_alloc (linux/mm/slub.c:2954)
> > > [   12.642458] irq event stamp: 42
> > > [   12.642460] hardirqs last enabled at (41): finish_task_switch (linux/./arch/arm64/include/asm/irqflags.h:35)
> > > [   12.642471] hardirqs last disabled at (42): cpuhp_thread_fun (linux/kernel/cpu.c:776 (discriminator 1))
> > > [   12.642476] softirqs last enabled at (0): copy_process (linux/./include/linux/lockdep.h:191)
> > > [   12.642484] softirqs last disabled at (0): 0x0
> > > [   12.642495] CPU: 0 PID: 24 Comm: cpuhp/0 Tainted: G        W         5.19.0-rc3-rt4-custom-piegon01-rt_0 #142
> > > [   12.642500] Hardware name: WIWYNN Mt.Jade Server System B81.03001.0005/Mt.Jade Motherboard, BIOS 1.08.20220218 (SCP: 1.08.20220218) 2022/02/18
> > > [   12.642506] Call trace:
> > > [   12.642508] dump_backtrace (linux/arch/arm64/kernel/stacktrace.c:200)
> > > [   12.642514] show_stack (linux/arch/arm64/kernel/stacktrace.c:207)
> > > [   12.642517] dump_stack_lvl (linux/lib/dump_stack.c:107)
> > > [   12.642523] dump_stack (linux/lib/dump_stack.c:114)
> > > [   12.642527] __might_resched (linux/kernel/sched/core.c:9929)
> > > [   12.642531] rt_spin_lock (linux/kernel/locking/rtmutex.c:1732 (discriminator 4))
> > > [   12.642536] ___slab_alloc (linux/mm/slub.c:2954)
> > > [   12.642539] __slab_alloc.isra.0 (linux/mm/slub.c:3116)
> > > [   12.642543] kmem_cache_alloc_trace (linux/mm/slub.c:3207)
> > > [   12.642549] __armpmu_alloc (linux/./include/linux/slab.h:600)
> > > [   12.642558] armpmu_alloc_atomic (linux/drivers/perf/arm_pmu.c:927)
> > > [   12.642562] arm_pmu_acpi_cpu_starting (linux/drivers/perf/arm_pmu_acpi.c:204)
> > > [   12.642568] cpuhp_invoke_callback (linux/kernel/cpu.c:192)
> > > [   12.642571] cpuhp_thread_fun (linux/kernel/cpu.c:777 (discriminator 3))
> > > [   12.642573] smpboot_thread_fn (linux/kernel/smpboot.c:164 (discriminator 3))
> > > [   12.642580] kthread (linux/kernel/kthread.c:376)
> > > [   12.642584] ret_from_fork (linux/arch/arm64/kernel/entry.S:868)
> > > 
> > > arm_pmu_acpi_cpu_starting() is called in the STARTING hotplug section,
> > > which runs with interrupts disabled. To avoid allocating memory and
> > > sleeping in this function, the pmu structures must be pre-allocated.
> > > 
> > > On ACPI systems, the count of PMUs is unknown until CPUs are
> > > hotplugged, cf:
> > > commit 0dc1a1851af1 ("arm_pmu: add armpmu_alloc_atomic()")
> > > 
> > > At most #PMU_IRQs pmu structures will be used and thus need to be
> > > pre-allocated.
> > > In arm_pmu_acpi_cpu_starting() subcalls, after checking the cpuid,
> > > decide to use or re-use a pre-allocated pmu structure. Thus the
> > > pre-allocated pmu struct can be seen as a pool.
> > > When probing, search and free unused pmu structures.
> > 
> > I think in retrospect I was trying to be too clever with
> > arm_pmu_acpi_cpu_starting() handling boot-time CPUs and late hotplug, and we
> > can make this simpler by handling the boot-time probing synchronously within
> > arm_pmu_acpi_probe(), removing a bunch of state.
> > 
> > I had a go at that, and in testing (in a QEMU TCG VM) atop arm64/for-next/core,
> > that seems to work (even with a faked-up heterogenous config). I've pushed that
> > to my `arm_pmu/acpi/rework` branch at:
> > 
> >    https://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git/log/?h=arm_pmu/acpi/rework
> >    git://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git arm_pmu/acpi/rework
> > 
> > ... does that work for you?
> 
> Thanks for the branch and for looking at this. I think there is an issue for late hotplug
> CPUs. Indeed the pmu structure allocation is done for the online CPUs at the
> time of probing. This let rooms for the case where none of the CPUs of a PMU is booted
> at startup.

The big problem here is that while we can detect those PMUs late, we only
register them with the core perf code in arm_pmu_acpi_probe(), so even if we
detect PMUs after that, those PMUs won't become usable.

I don't think we can support the case where none of the CPUs associated with a
PMU are booted at startup unless we make more substantial changes to the way we
register the PMUs with perf (and that would be going firther than what we
support with DT).

We can support bringing those CPUs online, just not registering them with perf.

> I tried the patch on a Juno-r2 with the 'maxcpus=1 apci=force' parameters. When late
> hotplugging CPU1 (which has a different pmu than CPU0), no pmu structure is found and
> the cpuhp state machine fails (since arm_pmu_acpi_cpu_starting() failed).

Ah, sorry, I missed that returning an error here would completely halt bringing
the CPU online. We arm_pmu_acpi_cpu_starting() to return 0 rather than -ENOENT
when it doesn't find a matching PMU, which would permit the CPU to come online.

I've made that change (and pushed that out to the branch), and it seems to work
for me, testing in a UEFI+ACPI VM on a ThunderX2, with the arm_pmu_acpi code
hacked to use the cpu index (rather than the MIDR) as the identifier for the
type of CPU.

With that change, booting a 64-vCPU VM with 'maxcpus=8', I see each of the
boot-time CPUs had its PMU registered:

| # ls /sys/bus/event_source/devices/
| armv8_pmuv3_0  armv8_pmuv3_3  armv8_pmuv3_6  software
| armv8_pmuv3_1  armv8_pmuv3_4  armv8_pmuv3_7  tracepoint
| armv8_pmuv3_2  armv8_pmuv3_5  breakpoint

... and if I try to online a non-matching CPU the CPU will come up, but I get a
notification that we couldn't associate with a PMU:

| # echo 1 > /sys/devices/system/cpu/cpu8/online
| Detected PIPT I-cache on CPU8
| GICv3: CPU8: found redistributor 8 region 0:0x00000000081a0000
| GICv3: CPU8: using allocated LPI pending table @0x0000000040290000
| Unable to associate CPU8 with a PMU
| CPU8: Booted secondary processor 0x0000000008 [0x431f0af1]

If I do the same thing but without the MIDR hack, it also seems to work:

| # ls /sys/bus/event_source/devices/
| armv8_pmuv3_0  breakpoint     software       tracepoint
| # cat /sys/bus/event_source/devices/armv8_pmuv3_0/cpus 
| 0-7
| # echo 1 > /sys/devices/system/cpu/cpu10/online 
| Detected PIPT I-cache on CPU10
| GICv3: CPU10: found redistributor a region 0:0x00000000081e0000
| GICv3: CPU10: using allocated LPI pending table @0x00000000402b0000
| CPU10: Booted secondary processor 0x000000000a [0x431f0af1]
| # ls /sys/bus/event_source/devices/
| armv8_pmuv3_0  breakpoint     software       tracepoint
| # cat /sys/bus/event_source/devices/armv8_pmuv3_0/cpus 
| 0-7,10

... so I think that should be ok?

Thanks,
Mark.

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